1. Field of the Invention
The present invention relates to a balanced acoustic wave filter device having a balance-unbalance conversion function. More particularly, the present invention relates to an acoustic wave filter device having an improved arrangement of ground lines on a piezoelectric substrate and an improved arrangement of ground terminals, and also relates to a composite filter using the acoustic wave filter device.
2. Description of the Related Art
A cellular phone, for example, has recently been proposed with a plurality of communication systems installed therein. Accordingly, there is a significant demand to increase the amount of attenuation in bands other than the pass band so as to reduce mutual adverse influences among the communication systems. Further, with downsizing of the cell phone, a small dual filter chip has been developed which includes two filter elements on one piezoelectric substrate.
As one example of such a dual filter chip, WO2006/003787 discloses a balanced acoustic wave filter device illustrated in FIG. 18.
A balanced acoustic wave filter device 1000 disclosed in WO2006/003787 includes an electrode structure on a piezoelectric substrate 1002. First and second longitudinally-coupled acoustic surface wave filters 1001 and 1001A are provided on the piezoelectric substrate 1002.
The first acoustic surface wave filter 1001 is a balanced acoustic surface wave filter having an unbalance terminal 1003 and first and second balance terminals 1004 and 1005, so as to provide a balance-unbalance conversion function. The acoustic surface wave filter 1001 includes first and second 3-IDT longitudinally-coupled resonator-type acoustic surface wave filters 1007 and 1008 which are connected to the unbalance terminal 1003 through a 1-port acoustic surface wave resonator 1006. Also, the acoustic surface wave filters 1007 and 1008 are connected to first and second balance terminals 1004 and 1005 through 1-port acoustic surface wave resonators 1009 and 1010, respectively.
The second acoustic surface wave filter 1001A is configured similar to the first acoustic surface wave filter 1001. Accordingly, the above description of the first acoustic surface wave filter 1001 is similarly applied to a description of the second acoustic surface wave filter 1001A with “A” added to each of reference numbers which are used in the description of the first acoustic surface wave filter 1001.
In the acoustic wave filter device 1000, the first and second acoustic surface wave filters 1001 and 1001A each having the balance-unbalance conversion function are disposed on one piezoelectric substrate 2. Further, a ground-side end of the balance-side IDT, i.e., the IDT connected to the balance terminal 1004 or 1005, in the acoustic surface wave filter 1001 and a ground-side end of the balance-side IDT connected to the first balance terminal 1004A or the second balance terminal 1005A in the second acoustic surface wave filter 1001A are connected in common by lines 1011 and 1011A and are further connected to a ground potential. Thus, a common ground terminal 1021 illustrated in FIG. 18 is connected to the ground potential, whereby the balance-side IDT is connected to the ground potential. In other words, the size of the acoustic wave filter device 1000 is reduced by connecting the ground-side ends of the IDTs, which are connected to the terminal ends 1004, 1005, 1004A and 1005A, in common to one ground terminal 1021.
However, the above-described acoustic wave filter device 1000 has problems in that a signal balance level is not sufficient between the first and second balance terminals 1004 and 1005 and between the first and second balance terminals 1004A and 1005A on the higher frequency side than the pass band, and an attenuation characteristic outside the pass band is not sufficient.